mysql 行转列列转行

一、行转列

即将原本同一列下多行的不同内容作为多个字段，输出对应内容。

建表语句

DROP TABLE IF EXISTS tb_score;

CREATE TABLE tb_score(
    id INT(11) NOT NULL auto_increment,
    userid VARCHAR(20) NOT NULL COMMENT \'用户id\',
    subject VARCHAR(20) COMMENT \'科目\',
    score DOUBLE COMMENT \'成绩\',
    PRIMARY KEY(id)
)ENGINE = INNODB DEFAULT CHARSET = utf8;

插入数据

INSERT INTO tb_score(userid,subject,score) VALUES (\'001\',\'语文\',90);
INSERT INTO tb_score(userid,subject,score) VALUES (\'001\',\'数学\',92);
INSERT INTO tb_score(userid,subject,score) VALUES (\'001\',\'英语\',80);
INSERT INTO tb_score(userid,subject,score) VALUES (\'002\',\'语文\',88);
INSERT INTO tb_score(userid,subject,score) VALUES (\'002\',\'数学\',90);
INSERT INTO tb_score(userid,subject,score) VALUES (\'002\',\'英语\',75.5);
INSERT INTO tb_score(userid,subject,score) VALUES (\'003\',\'语文\',70);
INSERT INTO tb_score(userid,subject,score) VALUES (\'003\',\'数学\',85);
INSERT INTO tb_score(userid,subject,score) VALUES (\'003\',\'英语\',90);
INSERT INTO tb_score(userid,subject,score) VALUES (\'003\',\'政治\',82);

查询数据表中的内容（即转换前的结果）

SELECT * FROM tb_score

先来看一下转换后的结果：

可以看出，这里行转列是将原来的subject字段的多行内容选出来，作为结果集中的不同列，并根据userid进行分组显示对应的score。

1、使用case...when....then 进行行转列

SELECT userid,
SUM(CASE `subject` WHEN \'语文\' THEN score ELSE 0 END) as \'语文\',
SUM(CASE `subject` WHEN \'数学\' THEN score ELSE 0 END) as \'数学\',
SUM(CASE `subject` WHEN \'英语\' THEN score ELSE 0 END) as \'英语\',
SUM(CASE `subject` WHEN \'政治\' THEN score ELSE 0 END) as \'政治\' 
FROM tb_score 
GROUP BY userid

2、使用IF() 进行行转列：

SELECT userid,
SUM(IF(`subject`=\'语文\',score,0)) as \'语文\',
SUM(IF(`subject`=\'数学\',score,0)) as \'数学\',
SUM(IF(`subject`=\'英语\',score,0)) as \'英语\',
SUM(IF(`subject`=\'政治\',score,0)) as \'政治\' 
FROM tb_score 
GROUP BY userid

注意点：

（1）SUM() 是为了能够使用GROUP BY根据userid进行分组，因为每一个userid对应的subject="语文"的记录只有一条，所以SUM() 的值就等于对应那一条记录的score的值。

假如userid =\'001\' and subject=\'语文\' 的记录有两条，则此时SUM() 的值将会是这两条记录的和，同理，使用Max()的值将会是这两条记录里面值最大的一个。但是正常情况下，一个user对应一个subject只有一个分数，因此可以使用SUM()、MAX()、MIN()、AVG()等聚合函数都可以达到行转列的效果。

（2）IF(`subject`=\'语文\',score,0) 作为条件，即对所有subject=\'语文\'的记录的score字段进行SUM()、MAX()、MIN()、AVG()操作，如果score没有值则默认为0。

3、利用SUM(IF()) 生成列 + WITH ROLLUP 生成汇总行,并利用 IFNULL将汇总行标题显示为Total

SELECT IFNULL(userid,\'total\') AS userid,
SUM(IF(`subject`=\'语文\',score,0)) AS 语文,
SUM(IF(`subject`=\'数学\',score,0)) AS 数学,
SUM(IF(`subject`=\'英语\',score,0)) AS 英语,
SUM(IF(`subject`=\'政治\',score,0)) AS 政治,
SUM(IF(`subject`=\'total\',score,0)) AS total
FROM(
    SELECT userid,IFNULL(`subject`,\'total\') AS `subject`,SUM(score) AS score
    FROM tb_score
    GROUP BY userid,`subject`
    WITH ROLLUP
    HAVING userid IS NOT NULL
)AS A 
GROUP BY userid
WITH ROLLUP;

运行结果：

4、利用SUM(IF()) 生成列 + UNION 生成汇总行,并利用 IFNULL将汇总行标题显示为 Total

SELECT userid,
SUM(IF(`subject`=\'语文\',score,0)) AS 语文,
SUM(IF(`subject`=\'数学\',score,0)) AS 数学,
SUM(IF(`subject`=\'英语\',score,0)) AS 英语,
SUM(IF(`subject`=\'政治\',score,0)) AS 政治,
SUM(score) AS TOTAL 
FROM tb_score
GROUP BY userid
UNION
SELECT \'TOTAL\',SUM(IF(`subject`=\'语文\',score,0)) AS 语文,
SUM(IF(`subject`=\'数学\',score,0)) AS 数学,
SUM(IF(`subject`=\'英语\',score,0)) AS 英语,
SUM(IF(`subject`=\'政治\',score,0)) AS 政治,
SUM(score) FROM tb_score

运行结果：

5、利用SUM(IF()) 生成列，直接生成结果不再利用子查询

SELECT IFNULL(userid,\'TOTAL\') AS userid,
SUM(IF(`subject`=\'语文\',score,0)) AS 语文,
SUM(IF(`subject`=\'数学\',score,0)) AS 数学,
SUM(IF(`subject`=\'英语\',score,0)) AS 英语,
SUM(IF(`subject`=\'政治\',score,0)) AS 政治,
SUM(score) AS TOTAL 
FROM tb_score
GROUP BY userid WITH ROLLUP;

运行结果：

6、动态，适用于列不确定情况

SET @EE=\'\';
select @EE :=CONCAT(@EE,\'sum(if(subject= \\'\',subject,\'\\',score,0)) as \',subject, \',\') AS aa FROM (SELECT DISTINCT subject FROM tb_score) A ;

SET @QQ = CONCAT(\'select ifnull(userid,\\'TOTAL\\')as userid,\',@EE,\' sum(score) as TOTAL from tb_score group by userid WITH ROLLUP\');
-- SELECT @QQ;

PREPARE stmt FROM @QQ;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

运行结果：

7、合并字段显示：利用group_concat()

SELECT userid,GROUP_CONCAT(`subject`,":",score)AS 成绩 FROM tb_score
GROUP BY userid

运行结果：

group_concat()，手册上说明:该函数返回带有来自一个组的连接的非NULL值的字符串结果。比较抽象，难以理解。通俗点理解，其实是这样的：group_concat()会计算哪些行属于同一组，将属于同一组的列显示出来。要返回哪些列，由函数参数(就是字段名)决定。分组必须有个标准，就是根据group by指定的列进行分组。

结论：group_concat()函数可以很好的建属于同一分组的多个行转化为一个列。

三、列转行

建表语句：

CREATE TABLE tb_score1(
    id INT(11) NOT NULL auto_increment,
    userid VARCHAR(20) NOT NULL COMMENT \'用户id\',
    cn_score DOUBLE COMMENT \'语文成绩\',
    math_score DOUBLE COMMENT \'数学成绩\',
    en_score DOUBLE COMMENT \'英语成绩\',
    po_score DOUBLE COMMENT \'政治成绩\',
    PRIMARY KEY(id)
)ENGINE = INNODB DEFAULT CHARSET = utf8;

插入数据：

INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES (\'001\',90,92,80,0);
INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES (\'002\',88,90,75.5,0);
INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES (\'003\',70,85,90,82);

查询数据表中的内容（即转换前的结果）

SELECT * FROM tb_score1

转换后：

本质是将userid的每个科目分数分散成一条记录显示出来。

直接上SQL：

SELECT userid,\'语文\' AS course,cn_score AS score FROM tb_score1
UNION ALL
SELECT userid,\'数学\' AS course,math_score AS score FROM tb_score1
UNION ALL
SELECT userid,\'英语\' AS course,en_score AS score FROM tb_score1
UNION ALL
SELECT userid,\'政治\' AS course,po_score AS score FROM tb_score1
ORDER BY userid

这里将每个userid对应的多个科目的成绩查出来，通过UNION ALL将结果集加起来，达到上图的效果。

附：UNION与UNION ALL的区别（摘）：

1.对重复结果的处理：UNION会去掉重复记录，UNION ALL不会；

2.对排序的处理：UNION会排序，UNION ALL只是简单地将两个结果集合并；

3.效率方面的区别：因为UNION 会做去重和排序处理，因此效率比UNION ALL慢很多；